Identify two individuals whose relatedness you want to calculate. Call them P and Q. For example's sake, suppose you want to calculate relatedness between an uncle and his niece. In this case, P is the uncle and Q is the niece.
Find their common ancestors. In this example, the father and mother of the uncle are also the niece's grandfather and grandmother. Note that there are two common ancestors. It arises from the obvious reason that it takes a man and a woman to reproduce.
Count the generational distance from P to Q for each ancestor. Starting with the uncle, go up the family tree once to reach his father and come down two steps to get to the niece. That equals a generational distance of three.
Repeat the process for the remaining common ancestor. In this example, the generational distance through the uncle's mother is also three.
Use the formula r = 0.5^n, where n represents the number of generational distance, to calculate relatedness. In this example, 0.5 ^3 equals 0.125. Since there are two common ancestors with identical generational distance, total relatedness between the uncle and the niece is 0.125 * 2, which equals 0.25.
Interpret relatedness as the percent of the genes the two relations share. Thus, a relatedness of 0.25 means that, on average, 25 percent of genes between the uncle and the niece are identical.
Calculate relatedness among your relations. Do you see a correlation between higher relatedness and altruistic behavior?